Solid fuel rockets are in widespread use due to the many advantages of working with solid propellants as opposed to liquid or gaseous fuels. Typically, solid fuel rocket motors provide the solid propellant within one or more annular metal rings, which are flexible to allow for temperature-induced expansion and contraction. Between the outermost annular ring and the exterior wall of the rocket motor a layer of insulation is provided, which is adhesively bonded to the inner surface of the external casing.
Previous insulation materials have been composed of a range of asbestos and non-asbestos-type materials. Representative of asbestos containing materials are the asbestos-filled phenolic resins (U.S. Pat. No. 3,243,956), neoprene or nitrile rubber having a solid additive such as asbestos, impregnated therein (U.S. Pat. No. 3,269,113), and a reaction product of tung oil with a phenolic resin, which contains a filler such as powdered asbestos (U.S. Pat. No. 3,990,369).
Due to the environmental and health concerns associated with the use of asbestos, various non-asbestos approaches to rocket motor insulation have been proposed in recent years. One approach employs two layers of material, the outermost layer being a sleeve composed of fiber glass impregnated with a high temperature resin, such as a phenolic resin, adjacent to which is provided a sheath composed of a high temperature rubber, such as Buna-N rubber (U.S. Pat. No. 3,928,965).
Also, a three component insulation wall assembly has been proposed, which contains a fire barrier layer, a middle thermal insulation layer, and an outer layer that provides structural stability. The fire barrier layer contains an ablative composite material within an ablative matrix material, such as a ceramic within a phenolic resin. The thermal insulation layer is made of a non-metallic honeycomb core material (U.S. Pat. No. 4,495,764).
Another approach is to employ an insulating adhesive to join the solid propellant to the rocket motor casing. A proposed material for use in this approach employs an epoxy resin, a chopped aramid (Kevlar) fiber pulp, and a microfine silicon dioxide filler (U.S. Statutory Invention Registration H1140). Relatedly, is an elastomeric insulating material comprising a crosslinked elastomeric polymer in which is dispersed a char-forming organic fiber, such as polyaramid pulp, and an inorganic particulate such as silica (U.S. Pat. No. 4,501,841). Hydrated alumina is mentioned as being an additive that can be used to enhance the flame-retardant properties of the insulation. U.S. Pat. No. 4,507,165 mentions that the polyaramid fibers can be replaced with cotton flock.
In yet another approach, a rocket motor insulation material is composed of a wound layer of insulation arranged about an inner solid propellant. The insulation layer contains a thermoplastic elastomer binder and is provided within an outer cylindrical casing formed of a composite material, such as graphite fibers and a thermoplastic elastomer binder. Representative of the thermoplastic elastomer binder is a polybutadiene/polystyrene block copolymer or fiberglass-reinforced polysulfone. A representative formulation for the insulation material contains thermoplastic elastomer binder at 57% by weight, alumina trihydrate at 13% by weight, other polybenzimidazole fibers at 16%-17% by weight, and lesser amounts of zinc borate and silene 732D. This particular arrangement of casing, insulation, and propellant, each of which contains thermoplastic elastomer binder, is reported to be effective in simultaneously fusing these layers to create a unitary motor structure (U.S. Pat. No. 5,388,399).
It is desired to develop a non-asbestos insulation material for use in rocket motors that is relatively simple to manufacture, has a low rate of char formation, and has improved resin curing properties. This object is met by the present invention.